Pulsed radar is a well known technique for detecting objects at a distance. A high-power radio frequency (RF), pulsed signal is transmitted in the direction of a target object, and the signal reflected off the object is detected by a sensitive receiver. The distance to the target may be determined by measuring the time delay between transmitting the signal and receiving signal reflected from the target. A velocity of the target may also be determined by measuring any frequency change of the reflected signal.
In a pulsed radar system used for detecting the presence of small objects at large distances, the detection sensitivity is limited by the amount of energy in the reflected pulse (also known as the return pulse) when it reaches the receiving antenna. The energy in the return pulse can be increased by increasing the pulse length. For a given transmission power and reception sensitivity, the detection sensitivity of the radar system can, therefore, be improved by increasing the length of the pulses in the pulsed signal. There is, however, a limit to how long the radar pulse can be made.
Because the target is small and far away, the return signal in the radar system is many orders of magnitude smaller than the transmitted signal. Furthermore, a transmission antenna can only be exactly impedance matched at a narrow band of frequencies, so that in any real radar system having a finite bandwidth signal, a small fraction of the transmitted signal is reflected back from the antenna along the radar's receive path in what is known as a return loss leakage signal.
If the radar system has multi-element, patch antennas, something that is highly desirable for radar beam steering, there is the additional problem of leakage from a transmitting element to the return path of one or more neighboring antenna elements, known as leakage due to mutual coupling.
In systems that are being used to locate small objects at a distance and that have collocated transmit and receive antennas, the transmitter is, therefore, turned off when the receiver is turned on, in order to avoid losing the faint return signal in the leakage signals. As a result, the length of the pulse that can be used in such systems is limited to the time an RF pulse takes to reach and return from the target. For a target at a distance of 300 km, the return time is about 500 msec.
One way to overcome this return time limit is to have separate transmit and receive antennas that are located sufficiently far apart that the transmitted signal does not couple into the receive antenna. The receiver can then operate at the same time as the transmitter and the pulse length is no longer limited by the return time.
In many practical situations, such as radar systems on aircraft or automobiles, it is not possible to separate the receiving and transmitting antenna sufficiently for them to operate at the same time. It is, therefore, highly desirable to have antennas, particularly multi-element, patch antennas, in which the magnitude of the return loss and mutual coupling leakage signals can be made sufficiently small that a weak return signal can be detected by the receiver even when the co-located transmitting antenna is being used at full power, thereby allowing very long pulse lengths to be used.